The wiring in most existing homes generally takes the form of a four-wire set, in which one pair of wires carries one telephone channel, and the other pair, if desired, carries a second channel.
Other types of buildings, such as commercial office buildings, contain similar telephone cable, which is used not only to support telephone service, as in a residence, but also to support a data network, in which computers communicate with each other. However, using the telephone cable to support the data network requires additional equipment, which may be expensive, and may require a system administrator for support.